Method and apparatus to reduce noxious components in the exhaust gases of internal combustion engines

ABSTRACT

The composition of exhaust gases from internal combustion engine is sensed, particularly the oxygen component thereof, and a sensed signal is derived, which is applied to a threshold detector, which triggers whenever the sensed signal passes a certain threshold value. The trigger signal controls and integrating controller to commence integrating, the integrating controller providing an output signal which is applied to set the air-fuel ratio such that the air number lambda is constantly controlled to be about 1. If integration by the integrating controller persists for a period of time in excess of a predetermined lapsed time, as determined by a pulse source controlled by speed of the engine, the integrating rate of the integrating controller is changed to provide for more rapid response when large changes have to be compensated.

United States Patent [191 Schmidt et a1.

[451 Jan. 1,1974

[73] Assignee: Robert Bosch GmbH,

Gerlingen-Schillerhohe, Germany 22 Filed: .lune2, 1972 211 Appl. No.: 259,254

[30] Foreign Application Priority Data Feb. 10, 1972 Germany P 22 06 276.6

[52] 11.8. C1 123/119 R, 123/140 MC, 60/276, 60/3928 T [51] Int. Cl. F02d 5/02 [58] Field of Search 123/140 MC, 119 R; 60/3928 T, 276

[56] References Cited UNITED STATES PATENTS 9/1972 Kendell 60/3428 T l/l962 Heppler ct al. 60/3928 T IC ENGINE 3,616,274 10/1971 Eddy 60/276 2,244,669 6/1941 Becker 123/140 MC 3,154,060 10/1964 Hundere 60/276 3,076,312 2/1963 Haigh 60/3928 T 3,070,735 12/1962 Kaiser et al. 60/3928 T 3,520,133 7/1970 Loft et al. 60/3928 T Primary ExaminerLaurence M. Goodridge Att0rneyFlynn & Frishauf [5 7] ABSTRACT The composition of exhaust gases from internal combustion engine is sensed, particularly the oxygen component thereof, and a sensed signal is derived, which is applied to a threshold detector, which triggers whenever the sensed signal passes a certain threshold value. The trigger signal controls and integrating controller to commence integrating, the integrating controller providing an output signal which is applied to set the air-fuel ratio such that the air number lambda is constantly controlled to be about 1. If integration by the integrating controller persists for a period of time in excess of a predetermined lapsed time, as determined by a pulse source controlled by speed of the engine, the integrating rate of the integrating controller is changed to provide for more rapid response when large changes have to be compensated.

13 Claims, 4 Drawing Figures PATENTEU JAN 1 I974 SHEET 1 0F 3 Fig.2

1C ENGINE PULSE Q GEN I v PATENTEDJAN :52;

SHEU 2 OF 3 METHOD AND APPARATUS TO REDUCE NOXIOUS COMPONENTS IN THE EXHAUST GASES OF INTERNAL COMBUSTION ENGINES CROSS REFERENCE TO RELATED PATENTS AND APPLICATIONS:

US. Pat. No. 3,483,851, Reichardt, Dec. 16, 1969; US. Ser. No. 259,157, filed June 2, 1972, Schmidt et al.; U.S. Ser. No. 259,134, filed June 2, 1972, Topp et al.; US. Ser. No. 265,547, filed June 23, 1972, Wahl et al.; US. Ser. No. 298,108, filed Oct. 16, 1972, Wahl et al. all assigned to the assignee of the present application. The present invention relates to an apparatus and to a method to reduce noxious components in the exhaust gases of internal combustion engines and more particularly to control the ratio of the mass of the air-fuel mixture applied to the internal combustion engine, that is, to control the air number lambda (A), by sensing the components of the exhaust gases and then controlling an integrating controller from the sensed signal.

Reference in the specification will be made to the air number, denoted lambda (A). This air number A is a measure of the composition of the air-fuel mixture. The number A is proportional to the mass of air and fuel, and the value of this number A is one (A 1.0) if' a stoichiometric mixture is present. Under stoichiometric conditions, the mixture has such a composition that, in view of the chemical reactions, all hydrocarbons in the fuel can theoretically combine with the oxygen in the air to provide complete combustion to carbon dioxide and water. In actual practice, even with a stoichiometric mixture, unburned non-combusted hydrocarbons and carbon monoxide are contained in the exhaust gases.

It is an object of the present invention to provide a method and an apparatus in which the inertia or lag of response of the equipment to carry out the control function is substantially eliminated.

Subject matter of the present invention: Briefly, the output signal from the sensing element is applied to a threshold detector. When the output signal passes a predetermined threshold level, a trigger signal is provided, the trigger signal being applied to an integrating controller to change the integrating direction of the control amplifier connected to control the fuel or air being applied to the engine and thus to change the composition of the fuel-air mixture.

The apparatus in accordance with the present invention is simple and inexpensive, and is reliable under the rough and varied operating conditions to which it can be subjected in motor vehicle use. The various electronic components of the control apparatus respond rapidly and with low dead intervals, and with low inertia, while still remaining stable.

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a timing diagram illustrating sensed signal voltage over a time 1 (line a) and the corresponding output (line b) of an integrating controller;

.FIG. 2 is a general block diagram of one embodiment of the invention;

P10. 3 is a general block diagram of another embodiment of the present invention; I

FIG. 4 is a more detailed schematic diagram of apparatus in accordance with P16. 2 to control A.

Curve a in FIG. 1 shows the sensed signal voltage from a sensor, such as an oxygen sensor 13 (HO. 2) exposed to the exhaust gases of an internal combustion engine. For a detailed description of the engine, and the sensing apparatus, reference is made to the cross referenced applications. Curve 0 clearly illustrates that upon change of the air number A about the value A 1, the output signal of the sensor 13 rapidly changes between limiting values. If the air number A changes by a relatively large amount, which may, for example, occur during acceleration of the internal combustion engine, then the output signal of the sensor will remain at one of its limits for a comparatively long period of time.

The resulting control voltage, for example the voltage derived from a control amplifier, is illustrated by curve b, in which the output voltage of a controller with integrating characteristic is shown. Each time that the sensor voltage goes through a zero value, the integration direction of the integrating controller changes. This causes the air number A to be always controlled in the direction of A 1. If, for example, the sensor voltage remains for a longer period of time at one limited value, as indicated by section 11,, then it would take a comparatively long period of time until the air number A is controlled back to have the value of A 1. The output signal of the controller is indicated by the section a of line b.

If it is determined that the output signal of the sensor has remained at a substantially constant value for a predetermined period of time then, in accordance with a feature of the invention, the integrating rate of the inte grating controller is changed. If, for example, the sensor has remained at one limited value for the time duration t then the integrating rate is changed to that illustrated in the broken curve b. This causes a greater change in the controlling effect, so that the sensor will more rapidly sense a change at air number A 1, namely at t,. Thus, the sensor voltage will reach already at the time period t a change to the other limiting.

value, as indicated in the broken line c of line a indicative of sensor voltage. Thus, by changing the time constant of the control amplifier, that is, by changing the integration rate in dependence on the number of changes of the output voltage of the sensor 13 through a certain threshold level, and controlling the timing constant, or integration rate of an integrating controller, the relative proportion of air and fuel applied to an internal combustion engine can be made much faster, so that the air number A is likewise controlled to have a value A 1 at a faster rate.

FIG. 2 illustrates an apparatus to carry out the present invention. Sensor 13, exposed to exhaust gases and sensing, for example, oxygen therein, is connected to an amplifier 12 which is in turn connected to a threshold detector or threshold switch 11. Threshold switch 1 1 has its output connected to an integrating controller-amplifier 10. The output of threshold detector 11 is additionally connected to a timing circuit 14, the out put from which is connected to the integrating controller 10 in order to adjust or set the integrating rate thereof. The internal combustion engine is only shown schematically at E, and the output shaft thereof is connected to a pulse generator 17 which provides pulses to an integrator 15, in turn connected to a threshold detector 16, elements 15 and 16 forming part of the timing circuit 14. Pulse generator 17 provides pulses of constant pulse duration but of a pulse repetition rate depending on engine speed. The output from the integrating controller is available at an output control bus 10.

Operation: Theexhaust gas sensor 13 provides an output signal, depending on the composition of the exhaust gases, which varies between well defined limits, as explained in detail in the cross referenced applications and more particularly in U.S. Ser. No. 259,157, the disclosure of which is herein incorporated by reference. The output signal is applied to amplifier l2 and then to threshold switch 1 1. Each time when the output signal from sensor 13 passes the threshold of the threshold switch, threshold switch 11 will provide an output trigger signal to change the direction of integration of the integrating controller 10. The characteristics of the integrating controller 10 are schematically illustrated within the block 10 of FIG. 2 in full line. Simultaneously with each change of the integration direction, integrator is re-set to zero. In a simple form, integrator 15 can merely be a capacitor which is discharged, to re-charge again with pulses derived from pulse generator 17. After re-set, integrator 15 integrates the pulses, of constant pulse duration, derived from pulse generator 17. This integration continues to the next trigger signal from threshold switch 11. Upon such a trigger signal, integrator 15 again is re-set to zero to commence integration over again, that is, the capacitor therein is discharged.

If the trigger signal from threshold switch 11 is not received within a reasonable time, that is, if the sensor 13 remains for a predetermined period of time at one limiting position as indicated, for example, by time period t, in FIG. 1, then the output signal of integrator 15 will reach a limit or threshold value of the second threshold switch 16. When threshold switch 16 is triggered, it provides a control signal, for example in form of a pulse to integrating controller 10 to change the time constant of the integrating controller 10 to a much faster integration rate as indicated in the broken line within block 10. The changed integration rate provides faster action of the controller, in accordance with broken line b (FIG. 1), and, due to the faster integration rate, the relationship of the masses of fuel and air of the fuel-air mixture can be regulated to a value of A l, or approximately 1, more rapidly.

FIG. 3 illustrates anotherembodiment of the system of the present invention to control the air number A. Similar elements have been given the same reference numerals and will not be described again. As before, sensor 13 applies its output signal over amplifier 12 to threshold switch 11 which triggers integrating controller 10. The timing circuit 18, in this embodimenqincludes a pulse generator 17 which is connected to a counter 19, having parallel outputs connected to a decode and trigger circuit 20. The threshold switch 11 is connected to counter 19; the output of decode and trigger circuit 20 is connected to the integrating controller 10 to change its integrating rate, when a trigger signal is derived from circuit 20. The output from integrating controller 10, over line 10' is applied to an air-fuel mixture controller C which controls the relative proportion of air and fuel over lines A, F, being applied to internal combustion engine E. The details of the control are set forth in the cross referenced applications. The engine itself is connected to the pulse generator 17 so that the pulses from pulse generator 17 will have a pulse repetition rate representative of engine speed.

Operation: Basically, the operation is the same as the embodiment of FIG. 2. When a trigger signal is derived from switch 11, the counter 19 is re-set to zero and starts to count at a counting rate determined by the pulses from pulse generator 17. When the counter reaches a predetermined counting state, as determined by decoding circuit 20, then decoding circuit 20 provides a trigger signal integrating controller 10 to change the time integration rate of the controller 10 so that the air number A will be controlled to a value of approximately 1 more rapidly.

FIG. 4 is a detailed block diagram of the circuit of the present invention to control the air number A. Amplifier 12, connected to sensor 13, includes an operational amplifier 21. Resistor 23 is connected between the output of operational amplifier 21 and the inverting input 22 thereof; the sensor 13 is connected over a coupling resistor 24 to the inverting input. The sensor 13 provides an output voltage characteristic of the composition of the air-fuel mixture, as sensed by analysis of the exhaust gases by sensor 13. The second input 25 of operational amplifier 21 is connected over a resistance 26 to the tap point of a voltage divider formed of resistors 27, 28 and connected between a common positive bus 29 and a common negative or chassis bus 30. The output of operational amplifier 21 has an output resistor 31, connected to positive bus 29 with its other terminal. The output is further coupled by means of coupling resistor 32 to the inverting input of an operational amplifier 33 which is part of the threshold switch 11. The second input of operational amplifier 33 is connected over coupling resistor 34 to a voltage divider formed of resistors 35, 36, connected between the positive and negative buses 29, 30. The output of operational amplifier 33 is connected over load resistor 37 to the positive bus 29 and, further, over coupling resistor 38 to the inverting input of operational amplifier 39 forming part of the integrating controller 10. The output of operational amplifier 39 is connected to the inverting input by means of a capacitor 40, which provides for the integrating characteristics of control amplifier 10. The non-inverting input of the operational amplifier is con nected by coupling resistor 41 to the tap point of a voltage divider formed of resistors 42, 43, connected between the supply buses 29, 30. The output of operational amplifier 39 is coupled to positive bus 29 by resistor 44, and is further connected to output terminal 10, for connection to a controller to control the mass relationship of the air-fuel mixture for the internal combustion engine.

The input resistor 38 to operational amplifier 39 has a parallel, shunt connection formed of the emittercollector path of a transistor 45 and a resistor 46. The control electrode of the resistor 45, which is a npn switching transistor, is connected over a coupling resistor 47 with the output electrode of a switching transistor 48, forming part of the timing circuit 14 (FIG. 2). Switching transistor 48 is connected to positive bus 29 over a coupling resistor 49, and has its emitter connected to common negative or chassis bus 30.

Transistor 48 is controlled over its base, by being connected to the end point of a voltage divider formed of resistors 50, 51, the tap point of which is formed by the output of an operational amplifier 52 forming part of the second threshold switch 16. The inverting input of operational amplifier 52 is connected over a coupling resistor 53 to a capacitor 54. The positive input of the operational amplifier 52 is connected to the tap point of a voltage divider formed of resistors 56, 57 which are connected across the positive and negative buses 29, 30. Capacitor 54 has a controllable variable resistor 58 in parallel thereto, and is connected over diode 59 and a resistor 60 to the output electrode of a switching transistor 61, which is further coupled to the positive bus 29 over a collector resistor 62. The emitter of transistor 61 is connected to negative bus 30; the control electrode, i.e. the base of transistor 61 is connected to a control circuit formed of resistor 63 connected to negative bus 30, a diode 64 and resistor 66, connecting to the positive bus 29. At the junction between the diode 64 and resistor 66, a coupling condenser 65 connects to a terminal which, in turn, is connected to pulse generator 17, not shown in FIG. 4. Capacitor 54 is connected parallel to the switching circuit of transistor 67, that is, in the emitter-collector path, which also includes a resistor 68. The base of transistor 67 is connected over a resistor 81 to the output electrode of a switching transistor 69, the collector of which is further connected over collector resistor 70 to positive bus 29. The emitter of transistor 69 is connected to the common chassis bus 30. Control electrode of switching transistor 69 is connected over resistor 71 to common positive bus 29 andfurther to two diodes 72, 73. Diode 73 connects to the junction point of a resistor 80 connected to common negative bus 30, and a capacitor 74 which is connected to the output of operational amplifier 33 of the first threshold switch 11. Diode 72 is connected to the junction point of a resistor 75, likewise connected to the common chassis bus 30, and a capacitor 76, which is connected to the collector of a transistor 77, the emitter-collector path of which is connected in series with a resistor 78 between positive and negative buses 29, 30. The control electrode of transistor 77 is connected over a resistor 79 to the output of operational amplifier 33.

Operation of the circuit in accordance with FIG. 4: The output signal of sensor 13 is amplified by amplifier l2 and applied to threshold detector 11. The integrating direction of integrating amplifier controller is changed in dependence on the output signal of operational amplifier 33 of threshold switch 11, by changing of the voltage applied to operational amplifier 39 over resistor 38. The time constant of the integrating pro cess of the operational amplifier 39 is determined by the value of the input resistance, in the present case the value of resistor 38. If the time constant is to be changed, then resistor 46 is placed in parallel to resistor 38 by rendering transistor 45 conductive. The conduction state of switching transistor 45 is controlled by the second threshold detector switch 16. This control depends on the charge state of capacitor 54. Capacitor 54 is charged over resistors 62, 60 and diode 59 when transistor 61 is blocked. The charge is interrupted when transistor 61 becomes conductive. Conduction of transistor 61 is controlled by pulses applied to the base of transistor 61 from the pulse source 17 over capacitor 65 and diode 64. These pulses have constant pulse duration, the pulse repetition rate, however, being proportional to engine speed. The output electrode of transistor 61 will have a negative voltage when transistor 61 becomes conductive, thus interrupting charging of capacitor 54. Resistor 58 connected in parallel to capacitor 54 discharges the capacitor 54 during the interval between pulses, the discharge current being determined by the resistance value of resistor 58. Since the pulse interval between pulses is greater at low speed than at high speed, capacitor 54 is discharged to a greater extent at low engine speed. At low speed, therefore, more charge pulses are ncessary to charge capacitor 54 to a predetermined level than at high speed. Thus, compensation of the speed-dependent processes in the exhaust system of the internal combustion engine is obtained automatically.

After a certain predetermined period of time, that is, after a certain number of charge pulses, in view of the discharge rate through resistor 58, the capacitor 54 will have a given charge thereon which corresponds to the switching limit of the second threshold switch 16. As soon as the threshold of operational amplifier 52 is reached, the amplifier 52 will provide a negative output signal which blocks the normally conductive transistor 48. When transistor 48 blocks, the base of transistor has a positive voltage applied thereto, controlling transistor 45 to conduct, and effectively placing resistor 46 in parallel to resistor 38, and thus changing the integration rate of operational amplifier 39.

Charge on the condenser is interrupted at each switch-over of the threshold switch 11, and capacitor 54 is again discharged. The discharge is effected over resistor 68 and the emitter-collector path of transistor 67. Transistor 67 is controlled to conduct when its base has a positive voltage applied thereto, which is provided by blocking of transistor 69. The conductionblocking switch-over characteristics of transistor 69 is determined by a negative signal applied to its base, which is transferred either by diode 72 or by diode 73. When the output signal of operational amplifier 33 becomes positive, transistor 77 will become conductive and a negative signal is transferred over capacitor 76 and diode 72 to the base of transistor 69. If a negative signal is derived from operational amplifier 33, it is directly transferred over capacitor 74 and diode 73. Thus, the condenser 54 is discharged at each change of integration direction of the integrating control amplifier 10. When, however, a switch-over of the threshold switch 11 is delayed over a longer period of time, then threshold switch 16 will respond and the time constant of the integrating control amplifier 10 is changed. As above described, the time which the controller requires to change the relative proportion of air and fuel to control the air number A to a value of )t approximately 1, is substantially reduced.

Various changes and modifications may be made within the inventive concept.

We claim: 1. Method to reduce noxious components in the exhaust gases of internal combustion engine comprising sensing the composition of the exhaust gases from the internal combustion engine and deriving an exhaust composition representative sensed signal;

analyzing the sensed exhaust composition signal to determine whether a characteristic thereof falls above or below a predetermined limit value;

integrating a representation of the sensed exhaust composition signal in a direction determined by whether the analyzed signal is above or below said limit value;

measuring the time during which the integrating step proceeds in a given direction;

changing the integration rate if the integration step has extended beyond a predetermined time period;

and controlling the composition of the air-fuel mixture being applied to the internal combustion engine in accordance with said integrated sensed sig nal.

2. Method according to claim 1, wherein the time measuring step comprises setting a time element at a datum level;

determining elapsed time by deviation of said element from the datum level and deriving a control signal after lapse of said predetermined period of time upon said deviation;

and utilizing said control signal to control the change of integration rate during said integration step.

3. Apparatus to reduce noxious components in the exhaust gases of internal combustion engines comprising means (13) sensing the composition of the exhaust gases from the internal combustion engine and deriving an exhaust composition representative sensed signal;

a first threshold detector (11) responding to said sensed signal and providing an output trigger signal when the sensed signal changes between levels above and below a predetermined threshold level; and

an integrating controller connected to control the relative proportion of air and fuel being applied to the internal combustion engine, the threshold detector having its output trigger signal connected to the integrating controller to change the direction of integration of the integrating controller, timing element (14, 18) triggered by the output trigger signal from the threshold detector and providing a timing output signal indicative of a predetermined time lapse, said timing signal being connected to the integrating controller to change the control characteristics of said controller after said predetermined time has elapsed;

said integrating controller providing an integrated output signal for control of said relative proportion of air and fuel in the mixture which is applied to the internal combustion engine.

4. Apparatus according to claim 3, wherein the integrating controller has a variable integration rate;

and the integration rate of the integrating controller is increased after the timing output signal has been applied thereto.

5. Apparatus according to claim 3, wherein the timing circuit (14) comprises a pulse source (17) providing cyclically recurring output pulses;

a second integrator (15) connected to integrate the timing pulses;

and a second threshold detector (16) supplying a trigger signal and forming said timing output signal when the second integrator has reached a predeter mined level.

6. Apparatus according to claim 5, wherein the pulse source (17) provides output pulses recurring at a rate proportional to speed of the internal combustion engme.

7. Apparatus according to claim 3, wherein the timing circuit (18) comprises a pulse source (17) providing cyclically recurring output pulses and a pulse counter means (19, 20) counting the pulses to a predetermined count and delivering said timing output signal at said predetermined count of the counter.

8. Apparatus according to claim 7, wherein the pulse source (17) provides output pulses recurring at a rate proportional to speed of the internal combustion engine.

9. Apparatus according to claim 4, wherein the integrating controller (10) comprises an operational amplifier (39) having a controllable input impedance (38, 46, 45), the value of the input impedance determining the integration rate.

10. Apparatus according to claim 9, wherein the input impedance of the operational amplifier (39) of the integrating controller comprises resistance means (38, 46) having two resistance values;

and controllable switching means (45) selectively placing the resistance means of one or the other value in circuit with the operational amplifier, the switching state of said controllable switching means being controlled by said timing signal.

11. Apparatus according to claim 5, wherein the threshold detector (16) comprises an operational amplifier (52);

an integrating capacitor (54) connected to the input of the operational amplifier (52);

a semiconductor switching element (61) controlling the charging of said capacitor (54) and a second semiconductor switching element (67) controlling the discharge of said capacitor (54), the state of said second semiconductor switching element being controlled by the output signal of said first threshold detector (1 1).

12. Apparatus according to claim 11, further comprising a current drain circuit (58) connected in parallel to said capacitor to permit flow of a controlled discharge leakage current.

13. Apparatus according to claim 3, further comprising a timing circuit connected to said integrating controller and sensing elapsed integrating time of said integrating controller;

said timing circuit providing a control signal to said integrating controller after a predetermined integrating interval has elapsed to change the integration rate of the controller to a higher integration rate. 

1. Method to reduce noxious components in the exhaust gases of internal combustion engine comprising sensing the composition of the exhaust gases from the internal combustion engine and deriving an exhaust composition representative sensed signal; analyzing the sensed exhaust composition signal to determine whether a characteristic thereof falls above or below a predetermined limit value; integrating a representation of the sensed exhaust composition signal in a direction determined by whether the analyzed signal is above or below said limit value; measuring the time during which the integrating step proceeds in a given direction; changing the integration rate if the integration step has extended beyond a predetermined time period; and controlling the composition of the air-fuel mixture being applied to the internal combustion engine in accordance with said integrated sensed signal.
 2. Method according to claim 1, wherein the time measuring step comprises setting a time element at a datum level; determining elapsed time by deviation of said element from the datum level and deriving a control signal after lapse of said predetermined period of time upon said deviation; and utilizing said control signal to control the change of integration rate during said integration step.
 3. Apparatus to reduce noxious components in the exhaust gases of internal combustion engines comprising means (13) sensing the composition of the exhaust gases from the internal combustion engine and deriving an exhaust composition representatIve sensed signal; a first threshold detector (11) responding to said sensed signal and providing an output trigger signal when the sensed signal changes between levels above and below a predetermined threshold level; and an integrating controller (10) connected to control the relative proportion of air and fuel being applied to the internal combustion engine, the threshold detector having its output trigger signal connected to the integrating controller to change the direction of integration of the integrating controller, a timing element (14, 18) triggered by the output trigger signal from the threshold detector and providing a timing output signal indicative of a predetermined time lapse, said timing signal being connected to the integrating controller to change the control characteristics of said controller after said predetermined time has elapsed; said integrating controller providing an integrated output signal for control of said relative proportion of air and fuel in the mixture which is applied to the internal combustion engine.
 4. Apparatus according to claim 3, wherein the integrating controller has a variable integration rate; and the integration rate of the integrating controller is increased after the timing output signal has been applied thereto.
 5. Apparatus according to claim 3, wherein the timing circuit (14) comprises a pulse source (17) providing cyclically recurring output pulses; a second integrator (15) connected to integrate the timing pulses; and a second threshold detector (16) supplying a trigger signal and forming said timing output signal when the second integrator has reached a predetermined level.
 6. Apparatus according to claim 5, wherein the pulse source (17) provides output pulses recurring at a rate proportional to speed of the internal combustion engine.
 7. Apparatus according to claim 3, wherein the timing circuit (18) comprises a pulse source (17) providing cyclically recurring output pulses and a pulse counter means (19, 20) counting the pulses to a predetermined count and delivering said timing output signal at said predetermined count of the counter.
 8. Apparatus according to claim 7, wherein the pulse source (17) provides output pulses recurring at a rate proportional to speed of the internal combustion engine.
 9. Apparatus according to claim 4, wherein the integrating controller (10) comprises an operational amplifier (39) having a controllable input impedance (38, 46, 45), the value of the input impedance determining the integration rate.
 10. Apparatus according to claim 9, wherein the input impedance of the operational amplifier (39) of the integrating controller comprises resistance means (38, 46) having two resistance values; and controllable switching means (45) selectively placing the resistance means of one or the other value in circuit with the operational amplifier, the switching state of said controllable switching means being controlled by said timing signal.
 11. Apparatus according to claim 5, wherein the threshold detector (16) comprises an operational amplifier (52); an integrating capacitor (54) connected to the input of the operational amplifier (52); a semiconductor switching element (61) controlling the charging of said capacitor (54) and a second semiconductor switching element (67) controlling the discharge of said capacitor (54), the state of said second semiconductor switching element being controlled by the output signal of said first threshold detector (11).
 12. Apparatus according to claim 11, further comprising a current drain circuit (58) connected in parallel to said capacitor to permit flow of a controlled discharge leakage current.
 13. Apparatus according to claim 3, further comprising a timing circuit connected to said integrating controller and sensing elapsed integrating time of said integrating controller; said timing circuit providing a control signal to said integrating controller after a predEtermined integrating interval has elapsed to change the integration rate of the controller to a higher integration rate. 